Fluid heating apparatus



1, 1965 M. J. M INERNEY ET AL 3,282,257

FLUID HEATING APPARATUS 4 Sheets-Sheet 1 Filed June 5, 1963 FIG] INVENTORS MICHAEL J. M -INERNEY JOSEPH L. CZYL WARREN HZQRY L ATTORNEY NOV. 1, 1966 J. MclNERNEY ETAL 3,282,257

FLUID HEATING APPARATUS Filed June 5, 1963 4 Sheets-Sheet 2 NM mm INVENTORS MICHAEL J- MEINERNEY JOSEPH L CZYL WARREN HENRY LAURIDZEN BY NORMAN A. WITT ATTORNEY Nov. 1, 1966 M. J. MCINERNEY ET AL 3,282,257

FLUID HEATING APPARATUS Filed June 5, 1963 4 Sheets-Sheet 5 INVENTORS MICHAEL J- M INERNEY JOSEPH L. CZYL WARREN HE Yb RI N ATTORNEY Nov. 1, 1966 J. cm ET AL 3,282,257 I FLUID HEATING APPARATUS Filed June 5, 1963 4 Sheets-Sheet 4 Q 9 LL.

3: INVENTORS MICHAEL J- M INERNEY JOSEPH L CZYL ATTO RN EY United States Patent 3,282,257 FLUID HEATING APPARATUS Michael J. Mclnerney, Highland Park, Joseph L. Czyl,

Harvey, and Warren H. Lauridsen, Bensenville, 111., assignors to Vapor Corporation, Chicago, 111., a corporation of Delaware Filed June 5, 1963, Ser. No. 285,796 Claims. (Cl. l22250) This invention relates in general to a fluid heating apparatus, and more particularly to a forced recirculated, multiple-coiled fluid heating apparatus, and still more particularly to a steam generator that may be quickly and easily modified to vary its capacity during the manufacture thereof. Further, this invention relates to a coil structure having a high efliciency that will permit the production of a line of fluid heating apparatuses that would normally require many different coil structures. While the invention will be disclosed in relation to a watertube steam generator, it will be appreciated that the invention is equally applicable to hot oil heaters, hot water heaters, and other hot fluid heaters.

Heretofore, the production of a full line of watertube steam generators having varying capacities has necessitated the manufacturer to provide several differently constructed watertube coils. Because of this, the cost of producing the line of generators was high and an unusually large inventory of parts was necessary to manufacture and service the line. Moreover, steam generators in the class of the present invention have required considerable floor and ceiling space and more square feet of heating surface per boiler horsepower (B.H.P.).

The steam generator of the present invention is a forced recirculated, multiple-coiled watertube unit that is horizontally fire having a drum that separates the water and steam. Water circulates from the drum by pumping it through the coiled tubes. Hot combustion gases from the combustor wipe over the outside surfaces of the coils counter to the flow of water that wipes the inside coil surfaces to thereby produce the highest possible heat transfer rate. This twin turbulence of hot gases and water provides a rated output of less than two square feet of heating surface per boiler horsepower.

Further, the structure of the present invention permits the use of only two different coils capable of producing at least six generator sizes which heretofore required twentyseven different coils. The watertube coils are connected in parallel and constructed to nest in such a manner that within a certain range any number of coils may be nested together to vary the capacity of the generator. Moreover, the nesting enables eflicient and high heat transfer action in a compact area and defines a continuous heat exchange surface.

Water circulated through the watertubes is drawn from the drum by action of a centrifugal pump, while a blower forces hot combustion gases from the combustion chamber of the burner or combustor over the heat exchange coil tube surfaces counter to the flow of water inside the coils. The forced circulation on both sides of the heat exchange surface provides the most efficient heat transfer possible. Water from the coils passes throught an outlet manifold into a steam lance in the drum. The water and steam are separated in the drum, whereby the steam is drawn from the drum and the separated and preheated water is recirculated through the watertubes at substantially saturation temperature.

It is therefore an object of the present invention to provide an improved fluid heating apparatus that is compact in size and capable of producing the highest possible heat transfer rate.

Another object of the present invention resides in the provision of a fluid heatingapparatus having a coil system that permits the production of several sizes of apparatuses with a small number of different coils.

Still another object of the present invention is in the provision of a forced recirculated, multiple-coiled watertube steam generator capable of producing full output from a cold start in a short period of time and capable of rapidly responding to fluctuating loads.

A further object of this invention is to provide a steam generator having a series of parallel-connected, nested coils of watertubes, wherein hot combustion gases wipe over the outside surfaces of the coils counter to the flow of water wiping the inside coil surfaces.

The coil of the present invention may be defined as a donut or annular coil that is wound from a continuous tube by the staggered winding technique into a multiple-row and multiple-layer tube bank having a low fluid pressure drop. In order to provide uniform heating through each coil, to encourage uniform flue gas flow and minimize unused heating surface, the coil has a rectangular crosssection. The coil is wound with the discharge or outlet pipe at the inner diameter and the inlet pipe at the outer diameter so that maximum sensitivity may be had where a coil overheat control is employed. However, it should be appreciated that the coil may be wound so that both ends are at the outer diameter or in any other suitable manner. The flue gases are directed from the center of the coil outwardly, thereby wiping the tubes successively from the innermost layer to the outermost layer. The spacing between the outer layers of each bank of tubes is less than the spacing between the inner layers to compensate for the cooler gases that wipe across the tubes and to increase the velocity of the gases and the Reynolds number for greater heat transfer action. This also prevents overload of the inner tube layers. A high effective total heating surface is produced since it is not necessary to place refractory between the coils but only at the ends thereof.

It is therefore, a further object of this invention to provide a coil for a fluid heating apparatus having an improved flexibility of usage since two coils can produce a line of apparatuses that heretofore normally would require twenty-seven different coils.

Another object of this invention is to provide a coil for a steam generator that is inherently thermally selfbalancing.

Still another object of this invention is in the provision of a coil for a watertube steam generator that is adaptable to be arranged in nested relationship to vary the capacity of a generator, and that will form a continuous multiplepass heat exchanger since each coil will mesh to any other of the same output and require no cement, baflles, or any other obstructions to flue gas flow or fluid flow.

A further object of the present invention resides in the provision of a coil for a watertube steam generator that is so constructed as to provide substantially equal and eflicient heat transfer action in all of the coils.

It is a still further object of this invention to provide a coil for a watertube steam generator having even flue gas distribution and radiant absorption, and thathas a low fluid pressure drop.

Another object of this invention is to provide a coil that is wound from a continuous tube by the staggered winding technique into a multiple-row, multiple-layer tube bank having a rectangular cross-section.

Still another object of this invention is to provide a coil that has a low heating surface to output ratio while the usable heating surface is kept at a maximum.

Other objects, features, and advantages of the invention will be apparent from the following detailed disclosure, taken in conjunction with the accompanying 3 sheets of drawings, wherein like reference numerals refer to like parts, in which:

FIG. 1 is a somewhat diagrammatic view of a steam generator constructed in accordance with the principles of the present invention;

FIG. 2 is an enlarged vertical sectional view taken through the combustor and coil housing of a steam generator;

FIG. 3 is an exploded perspective view showing the heat transfer coils of the present invention and a combustor;

FIG. 4 is a vertical sectional view taken substantially along line 44 of FIG. 3;

FIG. 5 is an enlarged cross-sectional view taken through a coil bank according to the invention;

FIG. 6 is a side elevational view of a coil bank of the present invention; and

FIG. 7 is a sectional view taken substantially along line 77 of FIG. 6.

Referring now to the drawings, and particularly to FIG. 1, a steam generator according to the present invention is illustrated which includes generally a heat exchanger 1%, a combustor or burner 11, and a steam separating drum 12. Water circulating through the heat exchanger it? is raised in temperature by the hot gases from the combustor 11, and the steam and water from the heat exchanger are separated in the steam separating drum 12. Both the combustor and drum are horizontally extending to thereby provide a horizontally fired steam generator.

The heat exchanger 10 includes a coil housing 13 having mounted therein a bank of coils including a plurality of watertube coils 14 15, 16 and 17 arranged together to provide a continuous heat exchange surface. The coils are annularly shaped and provided with inlets 14a, a, Ida and 17a, and outlets 14b, 15b, I61) and 17b. All of the inlets are connected to an inlet manifold 13, while all the outlets are connected to an outlet manifold 19. Both the inlet and outlet manifolds are connected to the steam separating drum E2, and more particularly, the outlet manifold is connected to a steam lance 20 by a pipe 21, while the inlet manifold is connected to the drum through a pipe 22. A recirculating water pump 23 is in the pipe 22 and serves to circulate the water through the watertube coils. The steam and water leaving the coils are separated in the drum 12 by the steam lance 20, whereby the drum thereafter defines a steam reservoir and a water reservoir. The drum, acting as a steam reservoir, will feed the load through a steam outlet 24, and while functioning as a water reservoir will permit preheated water at substantially saturation temperature to be recirculated through the return pipe 22 to the inlet manifold 18 and the coils.

A blowdown valve 25 is provided on the bottom of the drum 12 to permit sludge and sediment settling in the drum to be removed therefrom. A conventional safety valve 26 is also mounted on the drum. A feedwater line 27 is provided in the conventional manner to maintain the proper water level in the steam separating drum. It should also be appreciated that any other controls and/ or connections used in conventional drum operation may be provided in an actual installation, but such details are not important and significant as far as the present invention is concerned.

The coil housing 13 essentially encircles the watertube coils and defines with the outer periphery thereof a flue gas collecting chamber 23 that is also in annular form, and which communicates with a stack connection 29.

The hot combustion gases generated by the combustor 11 are forced into the hole or center opening 3t) of the banks of watertubes, radially outwardly to wipe across the surfaces of the watertube coils counter to the flow of water through the coils, into the flue gas collection chamber 28, and into the stack connection or fitting 29 and out the stack that is connected thereto.

To change the size of the steam generator, it is only necessary to add or subtract coils and change the size of the coil housing and the burner or combustor. Further, a larger or smaller size of coil may be provided but the same size coil would be employed in a bank where more than one is used in any one steam generator or fiuid heating apparatus. The coils 14, 15, 16 and 17 are constructed to nest with one another so that a substantially continuous heat exchange surface is provided. Further, these coils are connected in parallel to the inlet and outlet manifolds.

The combustor 11 includes generally a combustor housing 31 having mounted therein a fire pot 32 that defines a combustion chamber 33. This combustion chamber is closed at one end with a wall 34 that has a fuel burner 35 mounted centrally thereof to provide the combustion gases for the chamber. The other end of the fire pot 32 is provided with a choke 36 that defines a restricted choke opening 37 through which the hot combustion gases are discharged from the combustion chamber 33. An annular plenum chamber 3b surrounds the fire pot 32 and is connected to a blower 39 that pressurizes the air in the chamber and forces the air through openings around the fuel burner 35 to enhance the combustion in the combustion chamber 33 and to force the combustion gases through the coil banks 14, 15, 16 and 17 at a predetermined velocity. Either gas or oil may be employed as a fuel for the fuel burner 35.

Referring now particularly to FIGS. 2-7, it should be noted that each watertube coil is identical in configuration and size, whereby they nest with one another when assembled as shown in FIG. 2 to provide the heat exchange surface. Each coil is composed of a continuous tube wound with the staggering technique to define an annular shaped coil having a rectangular cross-section. Further, as he etofore mentioned, the tube is wound so that the inlet is at the outer diameter or stack side, while the outlet is at the inner diameter or combustion chamber side. The inner diameter or combustion side of each coil is aligned, while the outer diameter or stack side is aligned when the coils are in assembled relation as shown in FIG. 2. The coils as illustrated are wound to provide a threerow, seven-layer tube unit wherein the tubes of each row and layer are staggered with one another so that sinuous flue gas paths are defined between the inner and outer diameters of the coil.

Since the coils are constructed so that one coil nests with another in the manner shown particularly in FIG. 2, it is not necessary to provide any refractory between the coils. Accordingly, the effective usable heating surface is very high and a low heating surface to output ratio is obtainable. While the illustrated embodiment shows a three-row by seven-layer bank of tubes, it should be appreciated that this embodiment refers to a specific capacity and that other row-layer configurations may be employed in accordance with the invention.

The rectangular cross-section of the coil as shown par ticularly in FIG. 5 provides for uniform heating through each coil, encourages effective flue gas flow and minimizes unused heating surface.

The staggered rows of tubes or turns are separated and maintained in proper separated relationship by block spacers 50, FIGS. 6 and 7. While any number of block spacers may be arranged in circumferentially spaced relationship between each row of tubes, it is seen that effective separation is obtained by the use of three block spacers between the tubes of each layer. As seen particularly in FIG. 7, the block spacers 40 are welded to the adjacent tubes that are separated thereby. It is preferable that each coil is narrow as illustrated and deep so that when they are meshed one against the other, they will offer the same radiant area to the fiame or combustion gases where fired by a properly designed combustor thereby giving the desired result of having essentially equal flue gas flow through each coil. Thus, where each of the coils presents an equal parallel flue gas flow and each identical coil presents an equal fluid path length, the multi-coil heating exchangers of the present invention are self-balancing and require no orifices or dynamic balancers. Further, as long as each coil is identical, of rectangular cross-section, and wound from a tube multirows wide and multi-layers deep with each layer of tubes at a different pitch, the coils will mesh to form a continuous staggered tube heat exchanger.

The heat transfer ability of the flue gas is increased by gradually decreasing the spacing between the adjacent layers of tubes in a coil thereby increasing the overall efficiency of the coil. This is accomplished by mounting sinuous spacers 41, 42, 43, 44, 45 and 46 between each layer of tubes in each coil. The sinuous spacers at the inner diameter of each coil are larger than those adjacent the outer diameter to effectuate a decrease in spacing between the tube layers at the outer diameter or stack side. While each sinuous spacer 4146 may be gradually thinner to gradually decrease the spacing between the tube layers from the inner diameter to the outer diameter, it may be appreciated that some adjacent spacers may be of the same thickness. For example, the embodiment shown in FIG. 7 indicates the sinuous spacer 42 as being thinner than the spacer 41, while the spacers 43-46 are of the same thickness. The combustion gases are much cooler at the outer diameter of each coil than at the inner diameter, and therefore to obtain the greatest benefit from the gases, the outertube layers are more closely spaced together to decrease the flue gas path dimension and increase the velocity of the gases and the Reynolds number adjacent the outer diameter. An increase in heat transfer rate is effected by the increasing of the gas velocity and the Reynolds number. Thus, the maximum convective transfer capable in the outer tube layers of the coil, where the film temperature or gas-tube differential temperature and the radiant transfer are low, is provided with the present invention.

Wear pads 47 are secured to the outer lateral tubes of one layer on each coil as shown particularly in FIG. 6 to aid in properly spacing one coil from another. Pads 49, annular in shape, are provided at the opposite end walls as shown by the one end wall 50 in FIG. 5 to space the corresponding lateral faces of the outer tubes from the corresponding end wall and will prevent flue gas flow over the outer faces contacting the tube.

From the foregoing, it is seen that the present invention provides a fluid heating apparatus having a coil construction that enables many sizes of apparatuses to be made from a small number of different coils, and wherein a coil is capable of having a large heating surface with a low heating surface to output ratio, thereby defining a highly efficient and compact apparatus.

It will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present invention, but it is understood that this application is to be limited only by the scope of the appended claims.

The invention is hereby claimed as follows:

1. A steam generator comprising, a plurality of nested annularly-shaped, axially aligned and juxtaposed, watertube coils, each coil having a plurality of rows and layers of connected tubes and an inlet and an outlet, an inlet manifold being connected to the inlet of each coil, an outlet manifold being connected to the outlet of each coil, a steam separating drum connected to said manifolds, pump means for circulating water through said coils, a burner having a combustion chamber for delivering hot gases to the innermost layer of tubes in each coil, a flue gas collection chamber encircling said coils and communicating directly with the outermost layer of tubes of each coil, and a stock connection communicating with the flue gas collection chamber.

2. A steam generator comprising, a plurality of nested annularly-shaped, axially aligned and juxtaposed, watertube coils, each coil having a plurality of rows and layers of connected tubes and an inlet and an outlet, an inlet manifold being connected to the inlet of each coil, an outlet manifold being connected to the outlet of each coil, a steam separating drum connected to said manifolds, pump means for circulating water through said coils, a burner for generating hot gases, said burner having a combustion chamber in communication with said coils, blower means forcing the hot gases from the combustion chamber to the intermost layer of tubes in each coil and to wipe over the tubes, 21 flue gas collection chamber encircling said coils and communicating directly with the outermost layer of tubes of each coil, and a stock connection communicating with the flue gas collection chamber.

3. A steam generator comprising, a plurality of nested annularly-shaped, axially aligned and juxtaposed, watertube coils, each coil having a plurality of rows and layers of connected tubes and an inlet and an outlet, an inlet manifold being connected to the inlet of each coil, an outlet manifold being connected to the outlet of each coil, a steam separating drum connected to said manifolds, pump means for circulating water through said coils, a burner having a combustion chamber for delivering hot gases to the innermost layer of tubes in each coil, a coil housing encircling said coils and defining therewith a flue gas collection chamber, and a stack connection communicating with the flue gas collection chamber.

4. A fluid heating apparatus comprising, a plurality of annularly-shaped, axially aligned and juxtaposed, fluidtube coils, .said coils being identical and nested together to form a continuous heat exchange surface, each coil having a rectangular cross-section and being wound from a continuous tube so that the tubes of each row are staggered, each coil having a plurality of rows and layers of tubes and an inlet and an outlet, an inlet manifold connected to the inlet of each coil, an outlet manifold being con nected to the outlet of each coil, means for circulating fluid through said coils, a combustor for creating hot gases, said combustor having a fire pot in communication with said coils, and means forcing the hot gases from the fire pot to wipe over the tubes counter to the flow of fluid therethrough.

5. A fluid heating apparatus comprising, a plurality of annularly-shaped, axially aligned and juxtaposed, fluidtube coils, said coils being identical and nested together to form a continuous heat exchange surface, each coil having a rectangular cross-section and being wound from a continuous tube so that the tubes of each row are stag gered, means for spacing the tubes in each row and for spacing each layer of tubes, each coil having a plurality of rows and layers of tubes and an inlet and an outlet, an inlet manifold connected to the inlet of each coil, an outlet manifold being connected to the outlet of each coil, meansfor circulating fluid through said coils, a combustor for creating hot gases, said combustor having a fire pot in communication with said coils, and means forcing the hot gases from the fire pot to wipe over the tubes counter to the flow of fluid therethrough.

6. A fluid heating apparatus comprising, a plurality of annularly-shaped, axially aligned and juxtaposed, fluidtube coils, said coils being identical and nested together to form a continuous heat exchange surface, each coil having a rectangular cross-section and being wound from a continuous tube so that the tubes of each row are staggered, means for equally spacing the tubes apart in each row, means for gradually spacing each layer of tubes from the inner diameter to the outer diameter closer together, each coil having a plurality of rows and layers of tubes and an inlet and an outlet, an inlet manifold connected to the inlet of each coil, an outlet manifold being connected to the outlet of each coil, means for circulating fluid through said coils, a combustor for creating hot gases,

said combustor having a fire pot in communication with said coils, and means forcing the hot gases from the fire pot to wipe over the tubes counter to the flow of fluid therethrough.

7. A fluid heating apparatus comprising, a plurality of annularly-shaped, axially aligned and juxtaposed, fl-uidtube coils, said coils being identical and nested together to form a continuous heat exchange surface, each coil having a rectangular cross-section and being wound from a continuous tube so that the tubes of each row are staggered, means for equally spacing the tubes apart in each row, means for spacing the outer layers of tubes closer together than the inner layers, each coil having a plurality of rows and layers of tubes and an inlet and an outlet, an inlet manifold connected to the inlet of each coil, an outlet manifold being connected to the outlet of each coil, means for circulating fluid through said coils, a combustor for creating hot gases, said combustor having a fire pot in communication with said coils, and means forcing the hot gases from the fire pot to wipe out over the tubes counter to the flow of fluid there through.

8. A fluid heating apparatus comprising, a plurality of annularly-shaped, axially aligned and juxtaposed, fluidtube coils, said coils being identical and nested together to form a continuous heat exchange surface, each coil having a rectangular cross-section and being wound from a continuous tube so that the tubes of each row are staggered, block spacers for equally spacing the tubes apart in each row, sinuous spacers for spacing the layers of tubes apart, each coil having a plurality of rows and layers of tubes an an inlet and an outlet, an inlet manifold connected to the inlet of each coil, an outlet manifold being connected to the outlet of each coil, means for circulating fluid through said coils, a combustor for creating hot gases, said combustor having a fire pot in communication with said coils, and means forcing the hot gases from the fire pot to wipe over the tubes counter to the flow of fluid therethrough.

9. A fluid heating apparatus comprising, a plurality of annularly-shaped, axially aligned and juxtaposed, fluidtube coils, said coils being identical and nested together to form a continuous heat exchange surface, each coil having a rectangular cross-section and being wound from a continuous tube so that the tubes of each row are staggered, block spacers for equally spacing the tubes apart in each row, sinuous spacers for spacing the layers of tubes apart, said sinuous spacers spacing the outer layers of tubes apart being thinner than those spacing said inner layers apart, each coil having a plurality of rows and layers of tubes and an inlet and an outlet, an inlet manifold connected to the inlet of each coil, an outlet manifold being connected to the outlet of each coil, means for circulating fluid through said coils, a combustor for creating hot gases, said combustor having a fire pot in communication with said coils, and means forcing the hot gases from the fire pot to wipe over the tubes counter to the flow of fluid therethrough.

10. An annularly-shaped bank of watertube coils for a steam generator comprising, a tube of uniform cross section throughout wound to form a plurality of tubes in substantially radially extending rows and concentrically arranged layers having a substantially rectangular crosssection, separator means arranged between adjacent tubes for equally spacing the tubes from each other in each row, and separator means arranged between adjacent layer for spacing the layers of tubes apart so that the outer layers are closer together than the inner layers, thereby defining radially extending flue gas paths between the inner and outer layers that are smaller in cross-section at the outer layers wherein the flue gas flows through said paths from the inner layer to the outer layer.

11. An annularly-shaped bank of fluidtube coils for a fluid heating apparatus comprising, a tube of uniform cross section throughout wound to form a plurality of tubes in substantially radially extending rows and concentrically arranged layers having a substantially rectangular cross-section, the tubes in each layer being aligned and in each row being staggered, separator means arranged between adjacent tubes for equally spacing the tubes from each other in each row, and separator means arranged between adjacent layers for spacing the layers of tubes apart so that the outer layers are closer together than the inner layers, thereby defining radially extending sinuous flue gas paths between the inner and outer layers that are smaller in cross-section at the outer layers wherein the flue gas flows through said paths from the inner layer to the outer layer.

12. An annularly-shaped bank of fluidtube coils for a fluid heating apparatus comprising, a tube of uniform cross section throughout wound to form a plurality of tubes in substantially radially extending rows and concentrically arranged layers having a substantially rectangular cross-section, the tubes in each layer being aligned and in each row being staggered, separator means arranged between adjacent tubes for equally spacing the tubes from each other in each row, and separator means arranged between adjacent layers for spacing the layers of tubes apart so that the outer layers are closer together than the inner layers, thereby defining, radially extending sinuous flue gas paths between the inner and outer layers that are smaller in cross-section at the outer layers wherein the flue gas flows through said paths from the inner layer to the outer layer, an inlet at the outermost layer of tubes, and an outlet at the innermost layer of tubes.

13. In a steam generator, a plurality of annularlyshaped, axially aligned and juxtaposed, watertube coils arranged in nested relationship to provide a continuous heat exchange surface, each coil comprising, a tube of uniform cross section throughout wound to form a plurality of tubes in substantially radially extending rows and concentrically arranged layers having a substantially rectangular cross-section, the tubes in each layer being aligned and in each row being staggered, means for equally spacing the tubes from each other in each row, and means for spacing the layers of tubes apart so that the outer layers are closer together than the inner layers, thereby defining sinuous flue gas paths between the inner and outer layers that are smaller in cross-section at the outer layers wherein the flue gas flow direction is radially outwardly.

14. In a steam generator, a plurality of annularlyshaped, axially aligned and juxtaposed, watertube coils arranged in nested relationship to provide a continuous heat exchange surface, each coil comprising, a tube of uniform cross section throughout wound to form a plurality of tubes in substantially radially extending rows and concentrically arranged layers having a substantially rectangular cross-section, the tubes in each layer being aligned and in each row being staggered, means for equally spacing the tubes from each others in each row, means for spacing the layers of tubes apart so that the outer layers are closer together than the inner layers, thereby defining sinuous flue paths between the inner; and outer layers that are smaller in cross-section at the outer layers wherein the flue gas flow direction is radially outwardly, an inlet at the outermost layer of tubes of each coil, and an outlet at the innermost layer of tubes of each coil.

15. In a steam generator, a plurality of annularlyshaped axially aligned and juxtaposed, watertube coils arranged in nested relationship to provide a continuous heat exchange surface, each coil comprising, a tube of uniform cross section throughout wound to form a plurality of tubes in substantially radially extending rows and concentrically arranged layers having a substantially rectangular cross-section, the tubes in each layer being aligned and in each row being staggered, means for equally spacing the tubes from each other in each row, means for spacing the layers of tubes apart so that the outer layers are closer together than the inner layers, thereby defining sinuous flue gas paths between the inner and outer layers that are smaller in cross-section at the outer layers wherein the flue gas flow direction is radially outwardly, an inlet at the outermost layer of tubes of each coil, an outlet at the innermost layer of tubes of each coil, and means for connecting the inlets of the coils to an inlet manifold and the outlets of the coils to an outlet manifold, whereby the coils are parallel connected.

References Cited by the Examiner UNITED STATES PATENTS 10 2,201,625 5/1940 LaMont 122- 250 2,300,634 11/1942 Schoenfeld 122 250 2,805,048 9/1957 Angelery 122250X 5 FOREIGN PATENTS 686,378 1/1953 Great Britain.

ROBERT A. OLEARY, Primary Examiner.

10 KENNETH W. SPRAGUE, PERCY L. PATRICK,

Examiners.

FREDERICK L. MATTESON, JR., N. R. WILSON,

Assistant Examiners, 

10. AN ANNULARLY-SHAPED BANK OF WATERTUBE COILS FOR A STEAM GENERATOR COMPRISING, A TUBE OF UNIFORM CROSS ECTION THROUGHOUT WOUND TO FORM A PLURALITY OF TUBES IN SUBSTANTIALLY RADIALLY EXTENDING ROWS AND CONCENTRICALLY ARRANGED LAYERS HAVING A SUBSTANTIALLY RECTANGULAR CROSSSECTION, SEPARATOR MEANS ARRANGED BETWEEN ADJACENT TUBES FOR EQUALLY SPACING THE TUBES FROM EACH OTHER IN EACH ROW, AND SEPARATOR MEANS ARRANGED BETWEEN ADJACENT LAYER FOR SPACING THE LAYERS OF TUBES APART SO THAT THE OTHER LAYERS ARE CLOSER TOGETHER THAN THE INNER LAYERS, THEREBY DEFINING RADIALLY EXTENDING FLUE GAS PATHS BETWEEN THE INNER AND OUTER LAYERS THAT ARE SMALLER IN CROSS-SECTION AT OUTER 